A number of polythiophenes are known that may be useful as conductive or semiconductor materials in electronic devices such as thin film transistors, photovoltaic cells, organic/polymer light emitting diodes, and the like. Particularly useful polythiophenes are those that are soluble in organic solvents, and can thus be processed into microelectronic components by solution processes, such as spin coating, solution casting, dip coating, screen printing, stamp printing, jet printing and the like, thereby lowering the manufacturing cost of microelectronic devices. Specifically, certain polythiophenes, which contain repeating 2,5-thienylene (also known as 2,5-thiophendiyl) units possessing long side-chains, such as alkyl, arranged in a regioregular manner on the polythiophene backbone, may be suitable for these applications. The long alkyl side-chains, while imparting enhanced solubility characteristics to the polythiophenes, may also help induce and facilitate molecular self-organization when they are positioned in a regioregular manner on the polymer backbones.
In a condensed phase, such as in thin films, molecular self-organization of polymer molecules under appropriate conditions permits ordered microstructure domains, and which molecules when present in the charge transport layers of microelectronic devices could enhance their electrical performance. For example, for the polythiophene semiconductor channel layers in thin film transistors, the presence of the lamellar π-stacking microstructures has been known to lead to superior field-effect transistor properties.
Thin film transistors, which utilize solution processable organic/polymer materials and polymer composites, may also be fabricated on plastic substrates to permit lightweight structurally flexible integrated circuits that may be mechanically more robust and durable. These flexible lightweight integrated circuits are useful for incorporation into electronic devices, such as large-area image sensors, electronic paper and other display media where lightweight characteristics and device structural flexibility may be very appealing. These integrated circuit elements may also find use in low-end microelectronics, such as smart cards, radio frequency identification (RFID) tags, and memory/storage devices that require mechanical durability for extended life. For these applications, the performance of the polymer semiconductor materials, such as the polythiophenes in the channel layer, is of value. Also, while different synthetic methodologies and reaction conditions may provide analytically similar polythiophenes, the electrical performance of these polythiophenes, particularly their field-effect transistor characteristics when used as semiconductor channel materials in thin film transistor devices, may be dissimilar, for example there may be variations in the field-effect.
Certain polythiophenes have been reported for use as semiconductor materials in thin film field-effect transistors. One known example is a regioregular poly(3-alkylthiophene), see for example reference Z. Bao et al., “Soluble and processable regioregular poly(3-hexylthiophene) for field-effect thin film transistor application with high mobility,” Appl. Phys. Lett., Vol. 69, p4108 (1996), which is herein incorporated in its entirety by reference. The use of polymer semiconductors, such as polythiophenes, as the semiconductor channel layers has enabled the fabrication of flexible transistors on plastic substrates.
Polythiophenes can be prepared by many synthetic procedures depending specifically on the nature of the desired polythiophenes structures. A review of the chemistry and synthesis of polythiophenes was published by Richard D. McCullough, see reference R. D. McCullough, Adv. Mater., Vol. 10, p. 93 (1988), which is herein incorporated in its entirety by reference. Of all the preparative procedures for soluble polythiophenes, such as poly(alkylthiophenes), one synthetic methodology is metal halide-mediated oxidative coupling polymerization, reported by R. Sugimoto, see K. Yoshino, S. Hayashi, R. Sugimoto, “Preparation and Properties of Conducting Heterocyclic Polymer Films by Chemical Method,” Jpn J. Appl. Phys., Vol. 23, p. L899 (1984), and R. Sugimoto, S. Takeda, H. B. Gu, and K. Yoshino, “Preparation of soluble Polythiophene derivatives utilizing transition metal halides as catalysts and their property,” Chem. Express, Vol. 1, p. 635 (1986), each of which are herein incorporated in their entirety by reference. In this method, alkylthiophene is usually treated with ferric chloride (FeCl3) in chloroform under a blanket of dry air, or with a slow stream of dry air or inert gas bubbling through the reaction medium to drive off the generated HCl for a period of from a few hours to days. A detailed study of this polymerization was also reported by Leclerc, see reference M. Leclerc, F. M. Diaz, G. Wegner, “Structural analysis of poly(3-alkylthiophene)s,” Makromol. Chem., Vol. 190, p. 3105 (1989), which is herein incorporated in its entirety by reference.
Polythiophenes prepared with chloroform and other reaction media, such as for example, toluene, carbon tetrachloride, pentane, hexane, and the like, are illustrated in, for example, V. M. Niemi, P. Knuuttila, J.-E. Osterholm, and J. Korvola, “Polymerization of 3-Alkylthiophens with FeCl3,” Polymer, Vol. 33, p. 1559 (1992) and J. Kowalik, L. M. Tolbert, S. Narayan, and A. S. Abhiraman, “Electrically Conducting Poly(undecylbithiophene)s. 1. Regioselective Synthesis and Primary Structure,” Macromolecules, Vol. 34, p. 5471 (2001), which are herein incorporated in their entirety by reference.
After the formation of a polythiophene, it may undergo soxhlet extraction. Soxhlet extraction is a time-consuming process that may take a week or more. In addition, it does not always result in polymer with sufficient mobility. Furthermore, it cannot easily be conducted on a large scale. Thus, a method for treating polythiophene that is shorter, more effective and can be conducted on larger scales is desired.